Electrodes for biosensors

ABSTRACT

The present disclosure relates to an electrode for measuring an analyte. The electrode includes a first base layer, a first electrode layer upon the first base layer, and a second base layer. The first electrode layer is arranged between the first base layer and the second base layer. The first base layer includes a conductive metal, a conductive metal alloy, or carbon. The first electrode layer includes ruthenium metal, a ruthenium based metal alloy, nickel metal, or a nickel based metal alloy. The first base layer is made of different elements than the first electrode layer. The first base layer is more conductive than the first electrode layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-provisional Application Serial No. 17/851,677, filed Jun. 28, 2022, which is a continuation of U.S. Non-provisional Application Serial No. 17/045,364, filed Oct. 5, 2020, which is a national stage entry of PCT/US2019/028073, filed Apr. 18, 2019, which claims priority to U.S. Provisional Pat. pplication Serial No. 62/659,407, filed Apr. 18, 2018, such applications being hereby incorporated by reference in their entirety.

BACKGROUND

The present disclosure relates to electrodes such as those found in biosensors; and methods for creating and using such electrodes, such as in biosensors. In some embodiments, the biosensors have one or more electrodes containing a non-noble metal alloy having desired mechanical and electrical properties, such as a ruthenium containing alloy in combination with elements such as aluminum (Al), chromium (Cr), copper (Cu), molybdenum (Mo), nickel (Ni), rhenium (Re), and tungsten (W). In other embodiments, the alloy is a nickel containing alloy in combination with elements such as aluminum (Al), gold (Au), chromium (Cr), copper (Cu), molybdenum (Mo), palladium (Pd), ruthenium (Ru), tantalum (Ta), or titanium (Ti). In particular embodiments, the electrodes include a metal alloy stack that comprises (A) a base layer made of a conductive metal or carbon and (B) an electrode layer made from ruthenium metal or a ruthenium-containing metal alloy or nickel metal or a nickel-containing metal alloy. It is to be appreciated that the present disclosure is also amenable to other like applications.

Biosensors can be used in several applications, such as for measuring the amount of an analyte (e.g., glucose) in a biological fluid (e.g., blood). Blood glucose monitoring is a valuable tool in the management of diabetes. Diabetes is a disease in which the body is unable to control tightly the level of blood glucose, which is the most important and primary fuel of the body. This is due to either the pancreas not producing enough insulin, or to the cells of the body not responding properly to the insulin produced. Patients with diabetes are encouraged to monitor their glucose levels to prevent hyperglycemia, as well as other long-term complications such as heart disease, stroke, kidney failure, foot ulcers, and eye damage. A glucose biosensor is an analytical device for detecting the analyte, glucose, in the blood. Although glucose biosensors have been devised based on potentiometry, amperometry, and colorimetry, to date most commercially available biosensors are amperometric biosensors. These biosensors use a redox enzyme (e.g., glutathione peroxidases (GPX), nitric oxide synthase (eNOS, iNOS, and nNOS), peroxiredoxins, super oxide dismutases (SOD), thioredoxins (Trx), and the like), as the biological component responsible for the selective recognition of the analyte of interest (e.g., glucose).

A biosensor of this type is a relatively small strip of laminated plastic that can be exposed to a biological sample such as blood. An important feature of the biosensor is that it is disposable and only used one time. The strip acts as a substrate for a reaction chamber and two electrodes, a reference electrode and a working electrode, which are connected to the reaction chamber. The glucose biosensor contains a reagent layer that is attached to the working electrode. The reagent layer includes the selective recognition component (i.e., the redox enzyme) as well as electron mediators.

An electron mediator is an artificial electron transferring agent that helps shuttle electrons from the redox enzyme to the electrode surface. The mediator does this by reacting with the reduced enzyme and then diffusing to the electrode surface. Examples of mediators include vinyl ferrocene (VFc) initiated by 2,2′-azobisisobutyronitrile (AIBN), osmium complexes, quinone, ferricyanide, methylene blue, 2,6-dichloroindophenol, thionine, gallocyanine, indophenol, combinations thereof, and the like.

The biological fluid sample is introduced into the reaction chamber of the glucose biosensor and the biosensor is connected to a measuring device such as a meter for analysis using the biosensor’s electrodes. The analyte (glucose) in the sample undergoes a reduction/oxidation reaction at the working electrode (where the redox enzyme is located) while the measuring device applies a biasing potential signal through the electrodes of the biosensor. The redox reaction produces an output signal in response to the biasing potential signal. The output signal usually is an electronic signal, such as potential or current, which is measured and correlated with the concentration of the analyte in the biological fluid sample.

Electrodes in such biosensors are typically made from expensive precious metals, such as silver, gold, palladium, or platinum. It would be desirable to develop new materials that can be used as electrodes in a biosensor that have additional advantages when used with specific enzyme/mediator systems. It would also be desirable if such materials did not include precious metals, which are costly.

BRIEF DESCRIPTION

The present disclosure relates to biosensors having electrodes formed from a multilayer stack, including (1) a ruthenium-based or nickel-based metal alloy layer and (2) a base layer made from a conductive metal or a conductive metal alloy or carbon. A reagent is disposed on the electrode, the reagent comprising specific combinations of an enzyme and an electron mediator. The resulting electrode has physical and/or electrical properties that are advantageous when used with the reagent. These properties can include thinness, safety during reaction with the redox reagent, electrical conductivity, and reactivity with the redox reagent.

Disclosed in some embodiments are electrodes formed from at least one base layer and at least one electrode layer. The electrode layer is made from ruthenium metal or metal alloys comprising ruthenium in combination with at least one additional alloying element or nickel metal or nickel alloys in combination with at least one additional alloying element. It is contemplated that the ruthenium-containing alloy or nickel-containing alloy may be a binary, ternary, or quaternary alloy. The at least one base layer is made from a high conductivity low-cost metal (which does not contain ruthenium) or from carbon, e.g. in the form of diamond-like carbon. The base layer(s) and the electrode layer(s) are made from different materials. The base layer may exhibit a greater conductivity than the electrode layer. The base layer may be a conductive metal such as Al, Ag, Cr, Mo, and W, or can be carbon. The base layer and the electrode layer directly contact each other.

In some particular embodiments, the electrode is formed from an electrode layer (E) sandwiched between two base layers (B), to create a B-E-B configuration. The two base layers can be made of the same or different material.

In yet other particular embodiments, the electrode is formed from a base layer sandwiched between two electrode layers, to create an E-B-E configuration. The two electrode layers can be made of the same or different material.

In some particular embodiments, the ruthenium-containing alloy may contain from about 5 atomic percent (at%) to about 95 at% ruthenium, including 50 at% or greater ruthenium; about 5 at% to about 45 at% ruthenium; from about 50 at% to about 95 at% ruthenium; about 55 at% to about 95 at% ruthenium; about 50 at% to about 65 at% ruthenium; from about 50 at% to about 60 at%; about 55 at% to about 75 at% ruthenium; from about 60 at% to about 70 at%; about 65 at% to about 85 at% ruthenium; from about 70 at% to about 80 at%; about 75 at% to about 95 at% ruthenium; from about 80 at% to about 90 at%; or about 85 at% to about 95 at% ruthenium. This is on the basis of the alloy totaling 100 at%.

In other particular embodiments, the ruthenium-containing alloy contains from about 95 atomic percent (at%) to less than 100 at% ruthenium, including about 95 at% to about 96 at% ruthenium; from about 96 at% to about 97 at% ruthenium; about 97 at% to about 98 at% ruthenium; and about 98 at% to about 99 at% ruthenium. The additional alloying element(s) are present in a total amount of greater than zero at% to about 5 at%. This is on the basis of the ruthenium-containing alloy totaling 100 at%.

The additional alloying element(s) are, in particular embodiments, selected from the group consisting of aluminum, chromium, copper, nickel, rhenium, and tungsten.

In particular embodiments, the ruthenium-containing alloy may contain ruthenium in combination with about 5 at% to about 95 at% of the additional alloying element(s), including about 55 at% to about 95 at%; about 5 at% to about 50 at%; about 35 at% to about 50 at%; from about 40 at% to about 50 at%; about 25 at% to about 45 at%; from about 30 at% to about 40 at%; about 15 at% to about 35 at%; from about 20 at% to about 30 at%; about 5 at% to about 25 at%; from about 10 at% to about 20 at%; or about 5 at% to about 15 at%; or from about 0 at% to about 10 at% of the additional alloying element(s). In particular embodiments when the ruthenium-based alloy is a ternary alloy, the weight ratio of the first alloying element to the second alloying element may be from about 1:1: to about 2:1.

In other particular embodiments, the ruthenium-containing alloy may contain ruthenium in combination with greater than zero to about 5 at% of the additional alloying element(s), including about 1 at% to about 2 at%; about 2 at% to about 3 at%; about 3 at% to about 4 at%; and from about 4 at% to about 5 at%. In particular embodiments when the ruthenium-based alloy is a ternary alloy, the weight ratio of the first alloying element to the second alloying element may be from about 1:1: to about 2:1.

In some particular embodiments, the ruthenium-containing alloy is a binary alloy of (a) ruthenium and (b) either chromium or tungsten. These binary alloys may comprise about 55 at% to about 85 at% ruthenium, remainder chromium or tungsten; or about 55 at% to about 65 at% ruthenium, remainder chromium or tungsten; or about 75 at% to about 85 at% ruthenium, remainder chromium or tungsten.

In other particular embodiments, the ruthenium-containing alloy is a binary alloy of (a) ruthenium and (b) aluminum. These binary alloys may comprise about 60 at% to about 70 at% ruthenium, remainder aluminum; or about 15 at% to about 25 at% ruthenium, remainder aluminum.

In some further particular embodiments, the ruthenium-containing alloy is a binary alloy of (a) ruthenium and (b) nickel. These binary alloys may comprise about 5 at% to about 25 at% ruthenium, remainder nickel.

In still some other particular embodiments, the ruthenium-containing alloy is a ternary alloy of (a) ruthenium and (b) nickel and aluminum. These binary alloys may comprise about 20 at% to about 55 at% ruthenium, remainder nickel and aluminum combined.

In other particular embodiments, the ruthenium-containing alloy is a ternary alloy of (a) ruthenium and (b) chromium and tungsten. These binary alloys may comprise about 20 at% to about 55 at% ruthenium, remainder chromium and tungsten combined.

In certain embodiments, the alloy is a nickel-containing alloy that may contain from about 20 atomic percent (at%) to about 95 at% nickel; about 55 atomic percent (at%) to about 95 at% nickel; about 55 at% to about 65 at% nickel; from about 55 at% to about 60 at%; about 55 at% to about 75 at% nickel; from about 60 at% to about 70 at%; about 65 at% to about 85 at% nickel; from about 70 at% to about 80 at%; about 75 at% to about 95 at% nickel; from about 80 at% to about 90 at%; or about 85 at% to about 95 at% nickel. This is on the basis of the alloy totaling 100 at%.

The additional alloying element(s) may include, in particular embodiments, metals selected from the group consisting of aluminum, gold, chromium, copper, molybdenum, palladium, ruthenium, tantalum, and titanium. The alloy may contain nickel in combination with about 5 at% to about 45 at% of the additional alloying element(s), including; about 35 at% to about 45 at%; from about 25 at% to about 45 at%; from about 30 at% to about 40 at%; about 15 at% to about 35 at%; from about 20 at% to about 30 at%; about 5 at% to about 25 at%; from about 10 at% to about 20 at%; or about 5 at% to about 15 at%; or from about 0 at% to about 10 at% of the additional alloying element(s). In particular embodiments when the nickel-based alloy is a ternary alloy, the weight ratio of the first alloying element to the second alloying element may be from about 1:1: to about 2:1.

In a different set of embodiments, the alloy comprises from about 20 at% to about 95 at% nickel. In more specific embodiments, the alloy is a binary alloy and comprises from about 5 at% to about 80 at% of the first alloying element, and the first alloying element is selected from the group consisting of aluminum, ruthenium, tantalum, and titanium. In other specific embodiments, the alloy is a ternary alloy and consists essentially of about 20 at% to about 95 at% nickel; the first alloying element; and a second alloying element; wherein the first alloying element and the second alloying element are each selected from the group consisting of aluminum, ruthenium, tantalum, and titanium, and together the first alloying element and the second alloying element comprise from about 5 at% to about 80 at% of the alloy.

In some further embodiments, the alloy is a binary alloy consisting essentially of (a) about 55 at% to about 95 at% nickel and (b) about 5 at% to about 45 at% of either aluminum, chromium, or ruthenium.

In other further embodiments, the alloy is a binary alloy consisting essentially of (a) about 55 at% to about 95 at% nickel and (b) about 5 at% to about 45 at% of either aluminum, copper, chromium, tantalum, or titanium.

In still other embodiments, the alloy is a binary alloy consisting essentially of (a) about 55 at% to about 95 at% nickel and (b) about 5 at% to about 45 at% of aluminum, chromium, or titanium.

In some particular embodiments, the alloy is a binary alloy consisting essentially of (a) about 45 at% to about 95 at% nickel and (b) about 5 at% to about 55 at% of ruthenium.

In yet other embodiments, the alloy is a ternary alloy consisting essentially of (a) about 20 at% to about 55 at% nickel, (b) about 20 at% to about 30 at% titanium, and (c) about 20 at% to about 30 at% tantalum.

In more different embodiments, the alloy is a ternary alloy consisting essentially of (a) about 20 at% to about 55 at% nickel, (b) about 20 at% to about 30 at% aluminum, and (c) about 20 at% to about 30 at% ruthenium.

In additional particular embodiments when the nickel-based alloy is a ternary alloy, the alloy comprises (a) nickel and (b) either aluminum, ruthenium, tantalum, or titanium. These ternary alloys may comprise from about 20 at% to about 95 at% nickel, the remainder aluminum, ruthenium, tantalum, or titanium.

In some particular embodiments, the alloy is a binary alloy of (a) nickel and (b) either aluminum, chromium, or ruthenium. These binary alloys may comprise about 55 at% to about 95 at% nickel, remainder aluminum, chromium, or ruthenium.

In some particular embodiments, the alloy is a binary alloy of (a) nickel and (b) either aluminum, copper, chromium, tantalum, or titanium. These binary alloys may comprise about 55 at% to about 95 at% nickel, remainder aluminum, copper, chromium, tantalum, or titanium.

In some other particular embodiments, the alloy is a ternary alloy combining about 20 at% to about 55 at% nickel with about 20 at% to about 30 at% titanium and about 20 at% to about 30 at% tantalum.

In some other particular embodiments, the alloy is a ternary alloy combining about 20 at% to about 55 at% nickel with about 20 at% to about 30 at% aluminum and about 20 at% to about 30 at% ruthenium.

In some particular embodiments, the alloy is a binary alloy of (a) nickel and (b) either aluminum, chromium, or titanium. These binary alloys may comprise about 55 at% to about 95 at% nickel, remainder aluminum, chromium, or titanium.

In some particular embodiments, the alloy is a binary alloy of (a) nickel and (b) ruthenium. These binary alloys may comprise about 45 at% to about 95 at% nickel, remainder ruthenium.

In still some other embodiments, the base layer is made of carbon or a metal selected from the group consisting of Al, Ag, Cr, Mo, and W.

In some embodiments, the base layer is from about 10 nm to about 75 nm thick. The electrode layer may also be from about 10 nm to about 75 nm thick.

In further embodiments, the electrical conductivity of the base layer is greater than the electrical conductivity of the top layer.

Also disclosed herein are biosensors and articles comprising such multi-layer stacks with a ruthenium or nickel -containing alloy electrode layer and a conductive base layer. The base layer may increase the conductivity of the electrode.

Further disclosed in various embodiments are methods of creating a biosensor, including forming a first electrode by depositing a base layer on the surface of a substrate and then depositing an electrode layer upon the base layer. The base layer has a top surface and is formed from (1) carbon or (2) a conductive metal that does not contain ruthenium or (3) a conductive metal alloy that does not contain ruthenium. The electrode layer is formed from ruthenium metal or a ruthenium-containing metal alloy or nickel metal or a nickel-containing metal alloy. The ruthenium-based or nickel-based alloys can include any of the aforementioned alloys.

The methods can further comprise forming the first electrode by co-sputtering.

The methods can additionally comprise forming a reaction chamber in the substrate, the reaction chamber contacting the first electrode. In addition, a reagent layer can be formed on the first electrode to form a working electrode.

The methods contemplate that the first electrode operates as a reference electrode, and further comprise forming a second electrode on the substrate having the same structure as the first electrode (i.e. a multi-layer stack), and placing a reagent layer on the second electrode to form a working electrode.

These and other non-limiting characteristics of the disclosure are more particularly disclosed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings, which are presented for the purposes of illustrating the exemplary embodiments disclosed herein and not for the purposes of limiting the same.

FIG. 1A is a side view of an exemplary electrode of the present disclosure. A base layer contacts an electrode layer. The base layer can also be made from a combination of sublayers, if desired.

FIG. 1B is a side view of another exemplary electrode of the present disclosure. Here, an electrode layer is sandwiched between two base layers.

FIG. 1C is a side view of another exemplary electrode of the present disclosure. In this embodiment, a base layer is sandwiched between two electrode layers.

FIG. 2 is a perspective view of an exemplary biosensor of the present disclosure.

FIG. 3 is an exploded view of the biosensor of FIG. 2 .

FIG. 4 is a flow chart illustrating an exemplary method of producing an electrode of the present disclosure.

FIG. 5 is a cyclic voltammetry (CV) graph for a baseline gold (Au) electrode. The y-axis is current in microamperes (µA), and the x-axis is potential in volts (V).

FIG. 6 is a plot of baseline reversibility of current redox versus current oxidation for a scan of the baseline gold electrode. The y-axis is current in microamperes (µA).

FIG. 7 is a plot of baseline reversibility of current redox versus current oxidation for a scan of a ruthenium electrode of the present disclosure. The y-axis is current in microamperes (µA).

FIG. 8 is a CV graph for one scan of the ruthenium electrode of the present disclosure. The y-axis is current in microamperes (µA), and the x-axis is potential in millivolts (mV).

FIG. 9 is a CV graph for a second scan of the ruthenium electrode of the present disclosure. The y-axis is current in microamperes (µA), and the x-axis is potential in millivolts (mV).

FIG. 10 is a boxplot of current redox over six scans of the ruthenium electrode of the present disclosure. The y-axis is current in microamperes (µA).

FIG. 11 is a boxplot of potential redox over six scans of the ruthenium electrode of the present disclosure. The y-axis is potential in millivolts (mV).

FIG. 12 is a boxplot of current redox over six scans of a gold electrode for comparison with the boxplot illustrated in FIG. 10 . The y-axis is current in microamperes (µA).

FIG. 13 is a boxplot of potential redox over six scans of a gold electrode for comparison with the boxplot illustrated in FIG. 11 . The y-axis is potential in millivolts (mV).

FIG. 14 is an individual value plot of ruthenium, palladium, gold, and nickel peak separations for comparison to the Nernstian ideal of 59 mV. The y-axis is potential in millivolts (mV).

FIG. 15 is an individual value plot of current for various alloys of the present disclosure for comparison to known metals. The y-axis is current in microamperes (µA).

FIG. 16A is a contour plot showing the predicted resistance as a function of aluminum layer thickness and ruthenium-tungsten (RuW) layer thickness, based on a parallel resistor model. The y-axis and the x-axis are thickness in nanometers (nm), with resistance indicated in color (low is blue, high is green). The resistance is lowest along the upper left corner, and is highest at the lower left corner.

FIG. 16B is a contour plot showing the measured resistance as a function of aluminum layer thickness and ruthenium-tungsten (RuW) layer thickness. The y-axis and the x-axis are thickness in nanometers (nm), with resistance indicated in color (low is blue, high is green). The resistance is lowest along the upper left corner, and is highest at the lower left corner.

FIG. 17 is a cyclic voltammetry graph showing Cr/RuW for different thicknesses. The y-axis is current in amperes (A) and the x-axis is potential in volts (V). These values were measured at an initial potential of -300 mV; switching potential of 600mV; final potential -300 mV; scan rate of 50 mV/sec; and with a current full scale of 10 µA.

FIG. 18 is a cyclic voltammetry graph comparing three different electrode layer materials. The y-axis is current in amperes x 10⁻⁵, and the x-axis is potential in volts (V). RuW is dashed line, Au is dotted line, and RuW/C is solid line.

DETAILED DESCRIPTION

A more complete understanding of the components, processes and apparatuses disclosed herein can be obtained by reference to the accompanying drawings. These figures are merely schematic representations based on convenience and the ease of demonstrating the present disclosure, and are, therefore, not intended to indicate relative size and dimensions of the devices or components thereof and/or to define or limit the scope of the exemplary embodiments.

Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the disclosure. In the drawings and the following description below, it is to be understood that like numeric designations refer to components of like function.

The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.

References to “one embodiment,” “an embodiment,” or “embodiments” mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to “one embodiment”, “an embodiment”, or “embodiments” in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, step, etc. described in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, the present technology can include a variety of combinations and/or integrations of the features described herein.

As used in the specification and in the claims, the term “comprising” may include the embodiments “consisting of” and “consisting essentially of.” The terms “comprise(s),” “include(s),” “having,” “has,” “can,” “contain(s),” and variants thereof, as used herein, are intended to be open-ended transitional phrases, terms, or words that require the presence of the named ingredients/steps and permit the presence of other ingredients/steps. However, such description should be construed as also describing compositions or processes as “consisting of” and “consisting essentially of” the enumerated ingredients/steps, which allows the presence of only the named ingredients/steps, along with any impurities that might result therefrom, and excludes other ingredients/steps.

Numerical values in the specification and claims of this application should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of conventional measurement technique of the type described in the present application to determine the value.

All ranges disclosed herein are inclusive of the recited endpoint and independently combinable (for example, the range of “from 2 grams to 10 grams” is inclusive of the endpoints, 2 grams and 10 grams, and all the intermediate values).

A value modified by a term or terms, such as “about” and “substantially,” may not be limited to the precise value specified. The modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression “from about 2 to about 4” also discloses the range “from 2 to 4.” The term “about” may refer to ± 10% of the stated value.

It should be understood that when numerical ranges are provided, such ranges are to be construed as providing literal support for claim limitations that only recite the lower value of the range as well as claim limitations that only recite the upper value of the range. For example, a disclosed numerical range of 10 to 100 provides literal support for a claim reciting “greater than 10” (with no upper bounds) and a claim reciting “less than 100” (with no lower bounds).

The term “reagent” and variants thereof refers to a composition that may include multiple ingredients. For example, the reagent is sometimes used herein to describe a composition containing a redox enzyme, electron mediators, and additional substances / compounds. A reagent can be liquid or solid.

The present disclosure describes various layers formed “upon” another layer. When a first layer is described as being formed upon a second layer, the term “upon” should not be construed as requiring that the first and second layers directly contact each other (with no intervening layers). The term “upon” should be construed as describing the location of the first layer relative to the second layer.

The present disclosure is generally directed to a component for an electrode such as those used in a biosensor. As used herein, the term “biosensor” shall denote a device for analyzing biological samples. In some embodiments, the biosensor may be a medical sensor, such as a glucometer, and the biosensor component may comprise a test-strip for use with the biosensor. As used herein, the term “medical sensor” shall denote a biosensor used for medical monitoring and/or diagnosis.

The adjective “pure”, when used to describe a chemical element, is intended to refer to that element being in as pure a form as possible, and containing only unavoidable impurities that are unable to be economically removed.

The phrase “ruthenium metal” is used herein to refer to a metal that is made only of ruthenium, and may contain incidental impurities, i.e. contains 100 at% ruthenium.

The phrase “nickel metal” is used herein to refer to a metal that is made only of nickel, and may contain incidental impurities, i.e. contains 100 at% nickel.

A biosensor is typically formed from: (1) a substrate; (2) a pair of electrodes; and (3) a reagent layer that reacts with the analyte, and generally contains the redox enzyme and electron mediators.

In the present disclosure, at least one electrode is formed as a multi- layer stack. Put another way, the electrode is made from multiple layers of different materials. Specifically, the electrode includes (A) at least one base layer, and (B) at least one electrode layer in direct contact with the base layer. The electrode layer is made from either (1) ruthenium metal, or (2) from a metal alloy containing a quantity of ruthenium, or (3) nickel metal, or (4) from a metal alloy containing a quantity of nickel. In other words, the electrode layer contains at least ruthenium or nickel. The base layer is made from (1) a conductive metal which does not contain ruthenium or (2) carbon. The base layer and the electrode layer generally do not include precious metals such as gold, silver, palladium, or platinum. This makes the biosensor cheaper, which increases market opportunities for the biosensor. The ruthenium-containing or nickel-containing metal alloys can be used to provide physical and electrical property advantages when used with specific enzyme/mediator systems. Such physical and electrical properties may include thinness of the electrode, better electrical conductivity, stability over time, physical contact durability, lowered contact resistance for lowered/more consistent bias response, and/or better cohesion for finer line formation in circuitry. In particular embodiments, the top metal layer alloy is a ruthenium-containing alloy or a nickel-containing alloy.

FIG. 1A is a side view of an electrode 1 formed from a set of two layers. The electrode 1 includes a base layer 3 and an electrode layer 2 disposed on a first surface of the base layer. Generally, the base layer supports the electrode layer. In some embodiments, the base layer 3 is thicker than the top layer 2. The base layer and the electrode layer are made of different materials.

FIG. 1B is a side view of another exemplary embodiment of an electrode 1 formed from a set of three layers. Here, the electrode 1 includes an electrode layer 2 which is sandwiched between two base layers 3. Put another way, the electrode layer has opposite surfaces, each of which directly contact a base layer. The base layers and the electrode layer are made of different materials. It is contemplated that the two base layers may be made of the same material, or different materials.

FIG. 1C is a side view of another exemplary embodiment of an electrode 1 formed from a set of three layers. Here, the electrode 1 includes a base layer 3 which is sandwiched between two electrode layers 2. Put another way, the base layer has opposite surfaces, each of which directly contact an electrode layer. The base layer and the electrode layers are made of different materials. It is contemplated that the two electrode layers may be made of the same material, or different materials.

The base layer may have a thickness of from about 100 angstroms to about 100 nm. Preferably, the base layer may have a thickness of from about 10 nm to about 75 nm. The electrode layer may have a thickness of from about 100 angstroms to about 100 nm. Preferably, the electrode layer may have a thickness of from about 10 nm to about 75 nm. In some embodiments, the base layer 3 is thicker than the electrode layer 2. In other embodiments, the electrode layer is thicker than the base layer.

In some embodiments, the base layer 3 is composed of a conductive metal (ideally pure) that does not contain ruthenium, or a metal alloy that does not contain ruthenium, or carbon. In some embodiments, the base layer 3 has a higher electrical conductivity than the top layer 2. In some embodiments, the conductive material of the base layer 3 is a pure metal of, or an alloy that contains, an element selected from the group consisting of Cr, W, Ag, Al, Mo, Zn, Co or Ni. In some embodiments, the base material is either silver, aluminum, or chromium (ideally pure). It is also contemplated that the base layer could be made from a plurality or set of sublayers, wherein the sublayers are different metals. The carbon may be in the form of diamond-like carbon or a graphitic coating layer. The carbon can be sputtered as a thin film. The presence of the carbon helps maintain the conductivity of the overall electrode.

The electrode layer 2 is composed of ruthenium metal or a ruthenium-containing metal alloy or nickel metal or a nickel-containing metal alloy, as discussed further herein. It is specifically noted that the base layer and the electrode layer are made of different materials. In particular, nickel is identified as a suitable material for the base layer, but not when the electrode layer is made of nickel metal or a nickel-containing metal alloy. Nickel may be used in the base layer, for example, when the electrode layer is made of ruthenium metal or a ruthenium-containing metal alloy.

FIG. 2 is a perspective view of a biosensor 10. The biosensor 10 has a body 12, a fluid sampling end 14, an electrical contact end 16, and a vent opening 52. A notch 54 is disposed at the fluid sampling end 14 to facilitate loading of the fluid sample into the sample chamber 17. The fluid sampling end 14 includes a sample chamber 17 between a sample inlet 18 and the vent opening 52. The electrical contact end 16 has three discrete conductive contacts 16 a, 16 b, and 16 c.

FIG. 3 is an exploded view of the biosensor 10. The body 12 is composed of a substrate 20, an optional reagent holding layer 30, a channel forming layer 40, and a cover 50. The layers of the body 12 are generally made of plastics such as polyvinyl chloride, polycarbonate, polysulfone, nylon, polyurethane, cellulose nitrate, cellulose propionate, cellulose acetate, cellulose acetate butyrate, polyester, polyimide, polypropylene, polyethylene and polystyrene. Other polymer compositions known in the art include: nylon, polyesters, copolyesters, polyethylene, polypropylene, polyamides; polystyrene, polystyrene copolymers, styrene acrylonitrile copolymers, acrylonitrile butadiene styrene copolymers, poly(methylmethacrylate), acrylic copolymers, poly(ether-imides); polyphenylene oxides or poly(phenylene oxide)/polystyrene blends, polystyrene resins; polyphenylene sulfides; polyphenylene sulfide/sulfones; poly(ester-carbonates); polycarbonates; polysulfones; polysulfone ethers; and poly(ether-ketones); or mixtures of any of the other foregoing polymers. These materials may be either flexible or rigid, and should be generally non-conductive and chemically inert to the contemplated chemical reactions described herein.

The substrate 20 has a metal film 21 on which are delineated three electrodes 22, 24 and 26. The electrodes 22, 24, 26 may be formed by scribing or scoring the metal film 21, or by silk-screening electrodes 22, 24, 26 onto the substrate 20. Scribing or scoring of the metal film 21 may be done by mechanically scribing the metal film 21 sufficiently to create the three independent electrodes 22, 24, 26. The preferred scribing or scoring method of the present disclosure is done by using a carbon dioxide laser, a YAG laser or an excimer laser. Alternatively, the metal film is patterned as it is laid down, such that the metal film forms one electrode. Yet another method for forming an electrode for a biosensor comprises (a) providing a substrate; (b) providing a target; and (c) physical vapor depositing at least a portion of said substrate with material from said target to thereby form a conductive layer (i.e. electrode) on said substrate. Physical vapor deposition techniques include sputter coating (e.g., magnetron sputtering, unbalanced magnetron sputtering, facing targets sputtering, or the like), thermal evaporation, electron beam evaporation, laser ablation, arc vaporization, co-evaporation, ion plating, or the like. As illustrated here, three different films would be deposited to form the three electrodes 22, 24, 26.

The reagent holding layer 30 can be used when liquid reagents are desired to be used. The reagent holding layer 30 has three reagent holding openings 32, 34 and 36. The reagent holding opening 32 exposes a portion of the electrode 22, the reagent holding opening 34 exposes a portion of the electrode 24, and the reagent holding opening 36 exposes a portion of the electrode 26 creating reagent holding wells. This layer 30 is used to hold a sufficient quantity of chemical reagents in liquid form and to promote capillary action through the sample chamber of the sensor. The reagent holding layer 30 may be made from a plastic sheet and may be coated with a pressure sensitive adhesive, a photopolymer, ultrasonically-bonded to substrate 20, or silk-screened onto the substrate 20.

Usually, the channel forming layer 40 has a U-shaped cutout 42 located at the fluid sampling end 14. The length of the cutout 42 is such that when the channel forming layer 40 is laminated to reagent holding layer 30, electrode areas W and R are within the space defined by the cutout 42. The length, width and thickness of the U-shaped cutout 42 define the capillary channel volume.

The three reagent holding openings 32, 34, 36 define electrode areas W1, W2, and R, respectively, and hold chemical reagents forming two working electrodes and one reference electrode. Generally, the electrode areas are loaded with the reagent mixtures. The reagent mixtures for the working electrode areas 32, 34, 36 are a mixture of enzymes and redox mediators with optional polymers, surfactants, and buffers. A reference reagent matrix may be loaded in electrode area R that is similar to the reagent mixture of the working electrodes. It is contemplated that W1 and W2 use different enzymes / mediators, which can be used to check each other. Embodiments are also contemplated that have only one working electrode, which may be simpler to manufacture.

Alternatively, the chemical reagents can be used to form a reagent layer in the form of a dried solid film on the electrode areas W1, W2, R. In these embodiments, the reagent holding layer 30 is not needed.

Typically, electrode area R must be loaded with a redox reagent or mediator to make the reference electrode function. The reference reagent mixture preferably contains either oxidized or a mixture of an oxidized and reduced form of redox mediators, at least one binder, a surfactant and an antioxidant (if a reduced form of redox mediator is used) and a bulking agent. In the alternative, the reference electrode (electrode area R) could be also loaded with an Ag/AgCl layer (e.g. by applying Ag/AgCl ink or by sputter-coating an Ag or Ag/AgCl layer) or other reference electrode materials that do not require a redox mediator to function properly.

The size of the reagent holding openings is desirably as small as possible while still being capable of holding sufficient chemical reagent to function properly. As depicted here, the reagent holding openings are round and have a preferred diameter of about 0.03 in. (0.76 mm). The three reagent holding openings 32, 34, 36 are aligned with each other and are spaced about 0.025 in. (0.625 mm) from each other. The circular reagent holding openings are for illustrative purposes only and it should be understood that the shape of the reagent holding openings is not critical.

When a fluid sample is applied to a single strip of the present disclosure, the fluid sample enters the channel through the sampling end aperture and flows over W1, W2 and R and stops at the threshold of the vent opening. Chronoamperometry (i-t curve) can be used to measure the current response of the biosensor. Oxygen concentration (pO₂) can be controlled. Once a blood sample enters the strip, a potential of 0.3-0.5 volts is applied across the working electrodes and the reference electrode. The glucose concentration of the blood sample can then be measured.

The above described embodiments are based on amperometric analyses. Those skilled in the art, however, will recognize that a sensor of the present disclosure may also utilize coulometric, potentiometric, voltammetric, and other electrochemical techniques to determine the concentration of an analyte in a sample.

FIG. 4 is a flowchart that illustrates an exemplary method 100 of creating a biosensor with an electrode made from a multi-layer stack. The method 100 includes the steps of: forming multiple layers on the surface of a substrate, the multiple layers including at least one conductive electrode layer (E) and at least one base layer (B)on a surface of a substrate, in which the conductive electrode layer includes ruthenium or nickel (Step 120); forming a reaction chamber in the substrate in which the reaction chamber contacts the conductive electrode layer (Step 140); forming a reagent layer on the conductive electrode layer to form a working electrode (Step 160); and forming a second electrode on the substrate (Step 180).

At Step 120, an electrically conductive electrode layer and an electrically conductive base layer are formed on a surface of a substrate in which the conductive electrode layer includes ruthenium or nickel. In some embodiments, the base layer directly contacts the substrate, or put another way the base layer is located between the electrode layer and the substrate. In other embodiments, the electrode layer directly contacts the substrate, or in other words the electrode layer is located between the base layer and the substrate. In some instances, the layers of the multi-layer stack are sputtered onto a surface of a substrate. For example, a conductive base layer can be deposited onto the substrate surface by using fast ions to eject particles of the metal from a conductive material source due to contact of the metal source by energetic particles. Alternatively, the base conductive layer can be deposited by evaporation of a metal in a vacuum environment. The electrode layer can then be deposited onto the base layer using similar techniques. The multi-layer stack can be used to form a single electrode, or can be shaped or patterned to form two or more electrodes. In specific embodiments, the multi-layer stack includes a base layer and an electrode layer. As noted above, the base layer can be made from a plurality of sublayers, each sublayer being made from an electrically conductive metal that does not contain ruthenium. Similarly, the electrode layer can be made from a set of sublayers, one containing the ruthenium metal or alloy, and the other containing carbon.

It is realized that any configuration of layers disclosed herein may be formed at step 120, including but not limited to those depicted in FIGS. 1A-1C.

At Step 140, a reaction chamber is formed in the substrate in which the reaction chamber contacts the conductive electrode layer. The reaction chamber can be formed in the substrate by any method known in the art.

At Step 160, a reagent layer is formed on the conductive electrode layer of the metal layer stack to form a working electrode. The reagent layer can be formed on the conductive electrode layer by any method known in the art. More particularly, the reagent layer contains an enzyme, a coenzyme, and an electron mediator. Specific enzyme/mediator systems are contemplated. In a first system, the enzyme is glucose oxidase (GOD), the coenzyme is flavin adenine dinucleotide (FAD), and the mediator is hexacyanoferrate (ll)/hexacyanoferrate. In a second system, the enzyme is glucose dehydrogenase (GDH), the coenzyme is pyrroloquinoline quinone (PQQ), and the mediator is hexacyanoferrate (II)/hexacyanoferrate. In a third system, the enzyme is GDH, the coenzyme is PQQ, and the mediator is quinoneamine / phenylenediamine.

At Step 180, a second electrode is formed on the substrate. The second electrode can be formed as described in Step 120.

The electrode layer can be made from a ruthenium-containing metal alloy. The ruthenium-containing metal alloy itself can be a binary, ternary, or quaternary alloy of suitable metals. In particular embodiments, the ruthenium-containing metal alloy is a metal alloy containing ruthenium (Ru) in combination with one or more additional alloying element(s). The alloy may contain from about 5 atomic percent (at%) to about 95 at% ruthenium, including 50 at% or greater ruthenium; about 5 at% to about 45 at% ruthenium; from about 50 at% to about 95 at% ruthenium; about 55 at% to about 95 at% ruthenium; about 50 at% to about 65 at% ruthenium; from about 50 at% to about 60 at%; about 55 at% to about 75 at% ruthenium; from about 60 at% to about 70 at%; about 65 at% to about 85 at% ruthenium; from about 70 at% to about 80 at%; about 75 at% to about 95 at% ruthenium; from about 80 at% to about 90 at%; or about 85 at% to about 95 at% ruthenium.

It is particularly contemplated that the metal alloys containing ruthenium (Ru) are formed in combination with one or more elements such as aluminum (Al), chromium (Cr), copper (Cu), nickel (Ni), rhenium (Re), or tungsten (W). The alloy may contain from about 5 at% to about 95 at% of these additional alloying element(s), including about 55 at% to about 95 at%; about 5 at% to about 50 at%; about 35 at% to about 50 at%; from about 40 at% to about 50 at%; about 25 at% to about 45 at%; from about 30 at% to about 40 at%; about 15 at% to about 35 at%; from about 20 at% to about 30 at%; about 5 at% to about 25 at%; from about 10 at% to about 20 at%; or about 5 at% to about 15 at%; or from about 0 at% to about 10 at% of the additional alloying element(s).

As mentioned above, it is also contemplated that the electrode layer of the electrode used in the biosensor can be made from ruthenium metal (i.e. 100 at% ruthenium), or a metal alloy that contains a high content of ruthenium, such as from about 95 at% to less than 100 at% ruthenium.

The high content ruthenium metal alloy itself can be a binary, ternary, or quaternary alloy of suitable metals. In particular embodiments, the alloy is a metal alloy containing ruthenium (Ru) in combination with one or more additional alloying element(s). The alloy may contain from about 95 atomic percent (at%) to less than 100 at% ruthenium, including about 95 at% to about 96 at% ruthenium; from about 96 at% to about 97 at% ruthenium; about 97 at% to about 98 at% ruthenium; or about 98 at% to about 99 at% ruthenium.

It is particularly contemplated that the ruthenium-containing alloys contain a high content of ruthenium (Ru) in combination with one or more elements such as aluminum (Al), chromium (Cr), copper (Cu), nickel (Ni), rhenium (Re), or tungsten (W). The alloy may contain from greater than zero at% to about 5 at% of these additional alloying element(s), including about 1 at% to about 2 at%; about 2 at% to about 3 at%; about 3 at% to about 4 at%; and from about 4 at% to about 5 at% of the additional alloying element(s).

Any combination of ruthenium with one or more of the other elements at the compositional ranges specified above is contemplated. The metal layer stack may be formed using a cluster (i.e., co-sputtering) system with the substrate rotation disabled to allow a range of compositions over the wafer area. Desirably, one would fabricate a sputtering target for each layer in the metal layer stack, as this allows deposition uniformity to be maintained for each layer.

In particular embodiments, the ruthenium-containing alloy is a binary metal alloy containing ruthenium in combination with either chromium or tungsten. In further particular embodiments, the ruthenium-containing metal alloy is a binary alloy combining ruthenium (Ru) with aluminum (Al). In additional embodiments, the ruthenium-containing metal alloy is a binary alloy combining ruthenium (Ru) with nickel (Ni). In other additional embodiments, the ruthenium-containing metal alloy includes ruthenium (Ru) with rhenium (Re).

In some particular embodiments, the ruthenium-containing alloy is a binary metal alloy containing ruthenium in combination with either chromium or tungsten. These binary alloys may comprise about 55 at% to about 85 at% ruthenium, or about 55 at% to about 65 at% ruthenium, or about 75 at% to about 85 at%. The remainder of these binary alloys is chromium or tungsten.

In further particular embodiments, the ruthenium-containing metal alloy is a binary alloy combining ruthenium (Ru) with aluminum (Al). The alloy may contain from about 5 at% to about 45 at% ruthenium, or from about 55 at% to about 95 at% ruthenium, with the remainder being aluminum. In particular embodiments, the alloy may contain about 60 at% to about 70 at% ruthenium and about 30 at% to about 40 at% aluminum.

In additional embodiments, the ruthenium-containing metal alloy is a binary alloy combining ruthenium (Ru) with nickel (Ni). The alloy may contain from about 55 at% to about 95 at% ruthenium and from about 5 at% to about 45 at% nickel.

In other additional embodiments, the ruthenium-containing metal alloy includes ruthenium (Ru) with rhenium (Re). The alloy may contain from about 55 at% to about 95 at% ruthenium and from about 5 at% to about 45 at% rhenium.

The electrode layer can also be made from a nickel -containing metal alloy. The nickel-containing metal alloy itself can be a binary, ternary, or quaternary alloy of suitable metals. In particular embodiments, the alloy is a metal alloy containing nickel (Ni) in combination with one or more additional alloying element(s). The alloy may contain from about 55 atomic percent (at%) to about 95 at% nickel; about 55 at% to about 65 at% nickel; from about 55 at% to about 60 at%; about 55 at% to about 75 at% nickel; from about 60 at% to about 70 at%; about 65 at% to about 85 at% nickel; from about 70 at% to about 80 at%; about 75 at% to about 95 at% nickel; from about 80 at% to about 90 at%; or about 85 at% to about 95 at% nickel.

It is particularly contemplated that the alloys used herein are metal alloys containing nickel (Ni) in combination with one or more elements such as aluminum (Al), gold (Au), chromium (Cr), copper (Cu), molybdenum (Mo), palladium (Pd), ruthenium (Ru), tantalum (Ta), and titanium (Ti). The alloy may contain from about 5 at% to about 45 at% of these additional alloying element(s), including about 35 at% to about 45 at%; from about 40 at% to about 45 at%; about 25 at% to about 45 at%; from about 30 at% to about 40 at%; about 15 at% to about 35 at%; from about 20 at% to about 30 at%; about 5 at% to about 25 at%; from about 10 at% to about 20 at%; or about 5 at% to about 15 at%; or from about 0 at% to about 10 at% of the additional alloying element(s).

Any combination of nickel with one or more of the other elements at the compositional ranges specified above is contemplated. The alloy may be formed using a cluster (i.e., co-sputtering) system with the substrate rotation disabled to allow a range of compositions over the wafer area. Desirably, one would fabricate a sputtering target from the alloy, as this allows deposition uniformity to be maintained.

In some embodiments, the alloy is a metal alloy containing nickel (Ni) in combination with one or more additional alloying element(s). The alloy may contain from about 20 atomic percent (at%) to about 95 at% nickel. The one or more additional alloying element(s) can include aluminum, ruthenium, tantalum, and titanium. The alloy may contain from about 5 at% to about 80 at% of aluminum, ruthenium, tantalum, and titanium.

In some particular embodiments, the alloy is a binary metal alloy containing nickel in combination with either aluminum, chromium, or ruthenium. These binary alloys may comprise about 55 at% to about 95 at% nickel, or about 55 at% to about 85 at% nickel, or about 75 at% to about 85 at% nickel. The remainder of these binary alloys is aluminum, chromium, or ruthenium.

In further particular embodiments, the alloy is a binary metal alloy containing nickel in combination with either aluminum, copper, chromium, tantalum, or titanium. These binary alloys may comprise about 55 at% to about 95 at% nickel, or about 55 at% to about 85 at% nickel, or about 75 at% to about 85 at% nickel. The remainder of these binary alloys is aluminum, copper, chromium, tantalum, or titanium.

In some other particular embodiments, the alloy is a binary metal alloy containing nickel in combination with either aluminum, chromium, or titanium. These binary alloys may comprise about 55 at% to about 95 at% nickel, or about 55 at% to about 85 at% nickel, or about 75 at% to about 85 at% nickel. The remainder of these binary alloys is aluminum, chromium, or titanium.

In further particular embodiments, the metal alloy is a binary alloy combining nickel (Ru) with ruthenium (Ru). The alloy may contain from about 45 at% to about 95 at% ruthenium and from about 5 at% to about 55 at% ruthenium.

In some other particular embodiments, the alloy is a ternary alloy combining about 20 at% to about 55 at% nickel with about 20 at% to about 30 at% titanium and about 20 at% to about 30 at% tantalum.

In some other particular embodiments, the alloy is a ternary alloy combining about 20 at% to about 55 at% nickel with about 20 at% to about 30 at% aluminum and about 20 at% to about 30 at% ruthenium.

As one skilled in the art would readily appreciate, the metal alloys used in either the base layer or the electrode layer may comprise incidental impurities. As used herein, “incidental impurities” refer to any impurities that naturally occur in the ore used to produce the metal alloys or that are inadvertently added during the production process.

The resulting electrode(s) formed from the combination of a base layer and an electrode layer desirably exhibit improved physical and electrical properties, due to the use of ruthenium or nickel, either as a metal or as a metal alloy. One improved property is the thickness of the electrode, which can be very thin. In embodiments, the electrode can have a thickness of about 200 angstroms to about 200 nanometers, including from about 10 nanometers to about 100 nanometers. Another improved property is the electrical conductivity of the electrode, which can be less than 100 ohms/square (Ω/sq) at the desired thickness. The biosensor may also exhibit improved stability, as measured by electrochemical response stability over time when exposed to humidity and temperature variations, or as measured by changes in adhesion and/or abrasion differences when exposed to the reagent. Other desirable properties can include physical contact durability, lowered contact resistance for lowered/more consistent bias response, and/or better cohesion for finer line formation in circuitry. In addition, the resulting electrode(s) formed from the ruthenium-based or nickel-based alloy can be produced at a lower cost compared to more expensive metals such as gold.

As previously mentioned, the electrode can be formed by physical vapor deposition. This generally describes the coating of the substrate with the material from a target to form each layer of the electrode. As used herein, the term “physical vapor deposition” shall denote depositing thin-films by providing for the condensation of vaporized material onto a substrate. The physical vapor deposited coating may be performed with any type of physical vapor deposition process previously described, i.e., sputter coating, thermal evaporation, electron beam evaporation, laser ablation, arc vaporization, co-evaporation, ion plating, or the like. For example, in some embodiments, the physical vapor depositing step will be performed via a sputtering process, in which the substrate is coated with a layer by sputtering via a sputtering device. The resulting substrate with the multi-layer electrode coated thereon may be used as a biosensor component, which may include a working electrode, a reference electrode, or a counter electrode. In certain embodiments, such as when a roll of substrate material is vacuum coated with the layers of the electrode, via a roll-to-roll physical vapor deposition process, the resulting thin-film sheet may be cut apart to appropriate size to form a thin-film electrode upon a substrate. In other embodiments, the biosensor components can be formed from the thin-film sheet by etching, such as chemical or laser etching. In still other embodiments, the biosensor components can be formed using a patterned mask, which is laid on the substrate, and the conductive layers are physical vapor deposited thereover to form the biosensor component.

In certain specific embodiments, the biosensor components may be created via a roll-to-roll physical vapor deposition process that includes roll-to-roll magnetron sputtering. For instance, a substrate sheet comprising a polymer film made of PET (polyethylene terephthalate) with a thickness ranging from 25 µm to 250 µm and width of 33.02 cm may be sputtered using a 77.50 cm wide web roll-to-roll magnetron sputter coater. A single or a dual target configuration can be employed to deposit multiple layers of metal or metal alloys. A target comprised of a non-noble metal alloy plate can be used to form the base layer. A vacuum chamber of the sputter coater can be pumped down to base pressure of at least 10⁻⁵ Torr using a diffusion and mechanical pump combination. In other embodiments a combination of a mechanical pump, a turbo pump, a cryo pump, and/or an oil diffusion pump may be used. Magnetron sputtering cathodes housing the non-noble metal alloy targets having a generally rectangular shape of 15.24 cm x 30.48 cm can be energized using 2 KW power supplies (such as offered from Advanced Energy Inc.). An argon gas flow into the vacuum chamber can be controlled (such as via a MKS model 1179A flow controller) to set a sputtering pressure between 3 to 10 mTorr for use during the sputtering process.

A thickness and sheet resistance of each sputtered conductive layer can be efficiently controlled in-situ by controlling the roll-to-roll web speeds, i.e., controlling the speed of the substrate sheet as it travels through the vacuum chamber during sputtering. For example, for sputtering of a conductive layer of Composition A3, the web speed can be set to between 0.1 to 3.5 meters per minute and sputtering power density of between 2 to 8 Watts per square cm. As such, sputtered conductive layer of Composition A3 may be formed having a measured thickness value of about 25 nm and a sheet resistance of about 45 ohms per square.

EXAMPLES Example 1

The performance of existing biosensors utilizing electrodes made from precious metals, such as gold (Au) and palladium (Pd), was compared with the performance of electrodes made from ruthenium or ruthenium-based alloys disclosed herein. At the sputter stage, test samples of PET were cut to a 4-inch wafer shape. A conductive layer including ruthenium was sputtered using a DC magnetron (co-sputtering) sputter system, with the substrate rotation disabled to allow a range of compositions over the wafer area, and with argon gas. Target thickness was between about 300 and about 400 angstroms (A).

Cyclic Voltammetry (CV) analysis was selected to detect changes in performance, with a baseline performance based on pure gold electrodes. The CV test method measured faradaic current generated by the reduction of oxidation of substances near the electrode, including cathodic peak current (Ipc), anodic peak current (lpa), cathodic peak potential (Epc), and anodic peak potential (Epa). For combinatorial alloy deposition, an additional Si wafer was used to facilitate energy-dispersive X-ray spectroscopy (EDS) compositional analysis, or X-ray powder diffraction (XRD) characterization as required. Samples were handled and packaged using best practices to avoid contamination from contact with the sputtered area.

The test electrode samples were serialized within wafers to identify sample position, as well as serialized for wafer lot identification number. The initial test included at least 10 samples per wafer with locations distributed across the wafer. The Cyclic Voltammetry (CV) test parameters are summarized in Table 1 below:

TABLE 1 Initial CV test parameter (screening) summary Test Parameter Description Value Notes H₂SO₄ conc. Sulfuric acid cleaning solution concentration 0.1 M Used with potential scan to initially clean electrode of contamination. KCl conc. Potassium Chloride electrolyte concentration 0.1 M - K₄Fe(CN)₆ conc. Potassium ferrocyanide analyte concentration 1 mM - Sample volume Volume of tested solution 20 µL - Waveform / Scan range Potential range -0.5 V to 0.3 V - Scan rate Rate of potential change 50 mV/s Test repeats # of repeated scans (same electrode) 6 or greater Can abort scans after 3 if no redox response detected

Electrodes were singulated in strips using a paper cutter, then manually singulated from the strips with scissors. The electrodes were placed in a test fixture and connected to a BASi potentiostat and analyzer. Cyclic voltammetry (CV) was performed using 1 nM of K₄Fe(CN)₆ concentration in 0.1 M KCI. Peak current (IP), Ipc, Ipa, Epc, and Epa were recorded as well as a qualitative assessment.

First, the ferro/ferri electrochemical response of pure metal thin-film depositions was evaluated in the range of test parameters listed in Table 1 above and established for screening. The assessment of pure depositions is shown below in Table 2:

TABLE 2 Assessment of pure metal thin-film depositions Bulk Density (g/cc) Sputter rate (nm/s) Sputter yield *= Redox Background current (est.) (uA) Qualitative Notes 20.53 12 0.9 Re 1 Variable - smooth 19.35 8 0.6 W 3 Smooth response 19.31 32 2.8 Au* - Good redox 16.6 8.5 0.6 Ta 0.1 Flat 12.3 18 1.3 Ru* - Good redox, background current 12.02 27 2.4 Pd* - Good redox, changes over multiple scans 10.2 12 0.9 Mo 10 Smooth response, large starting current 8.92 32 2.3 Cu 1 Noisy, variable. Reaction at 100 mV 8.9 19 1.5 Ni* - Good redox, secondary reaction 8.9 19 1.4 Co 1 Variable - flat 8.57 8 0.6 Nb 1 Smooth response 7.86 18 1.3 Fe 10 Smooth response 7.3 80 In - 7.2 32 1.3 Cr 1 Oxidation peak 6.49 8.5 0.7 Zr 0 Flat 5.75 80 Sn - 4.5 8 0.6 Ti 0.1 Flat 2.7 17 1.2 Al 1 Variable - flat, noisy 2.37 - - B - -

Baseline variability was chosen to be based on gold electrodes for subsequent material response evaluation. The cyclic voltammetry of six randomly picked gold electrodes from the same sputtered wafer were equivalent enough for establishment of baseline electrode variability. The electrodes showed consistent characteristics at different locations on the wafer, and multiple scans indicated stabilization of the response over the multiple scans. As shown in FIG. 5 , the cyclic voltammetry using the gold electrode was a symmetrical cycle which suggested that the electrode was able to be used for the electrochemical test. After the first three scans, scans 4-6 demonstrate that the cyclic voltammetry response was stable and repeatable.

As an evaluation of baseline reversibility, the third scan for the gold electrode was evaluated for a significant difference in peak currents as shown in FIG. 6 , as a reversible system will result in lpa/lpc = 1. In this case, a significant difference in means was not detected, through Ipa/lpc = 1.05.

A T-Test was performed between the current redox and the current oxidation for the baseline gold electrodes to test the difference between the two samples. The results of the T-Test are shown below in Table 3:

TABLE 3 Two-Sample T-Test and Cl: current red and current ox (gold) Two-sample T for current red vs current ox N Mean StDev SE Mean Current red 21 1.693 0.161 0.035 Current ox 21 1.779 0.209 0.040 Difference = µ (current red) - µ (current ox) Estimate for difference: -0.0858 95% Cl for difference (-0.2022, 0.0306) T-Test of Difference = 0 vs ≠): T-Value = -1.49 P-Value= 0.144 DF=37

As shown in Table 3 above, a significant difference in means was not detected.

The significance of ruthenium mean or variance differences were based on the established gold baseline variability discussed above. Based on electrode pass/fail criteria of redox response detected with 1 mM potassium ferricyanide analyte between about -500 to about 500 mV, ruthenium-based alloys demonstrated good stability and electrochemical response compared with the baseline gold electrodes. The ruthenium redox response was stable and repeatable in the range tested. As demonstrated by FIG. 7 and the results of the T-Test in Table 4A below, there appeared to be a larger amount of non-faradaic (i.e., background) current compared to gold electrode systems, since lpa/lpc = 1.1 rather than the ideal case of lpa/lpc = 1, possibly indicating less of a reversible system than gold.

TABLE 4A. Two-Sample T-Test and Cl: current red and current ox (ruthenium) Two-sample T for current red vs current ox N Mean StDev SE Mean Current red 17 1.597 0.163 0.039 Current ox 17 1.753 0.144 0.035 Difference = µ (current red) - µ (current ox) Estimate for difference: -0.1562 95% Cl for difference (-0.2637, 0.0486) T-Test of Difference = 0 vs ≠): T-Value = -2.96 P-Value= 0.006 DF=31

However, the cyclic voltammetry using the ruthenium electrode in scan 2 (FIG. 8 ) and scan 6 (FIG. 9 ) were mostly symmetrical cycles, which suggests that the electrode was able to be used for the electrochemical test.

Moreover, as shown in FIGS. 10-13 , ruthenium results in an increasing concave down stabilization pattern which compares favorably with the gold electrode baseline.

The significance of nickel mean or variance differences were also based on the established gold baseline variability discussed above. Based on electrode pass/fail criteria of redox response detected with 1 mM potassium ferricyanide analyte between about -500 to about 500 mV, pure nickel demonstrated good stability and electrochemical response compared with the baseline gold electrodes. The nickel redox response was stable and reversible in the range tested. As demonstrated by the results of the T-Test in Table 4B below, the amount of non-faradaic (i.e., background) current appeared to be similar to gold electrode systems, since lpa/lpc ≈ 1 when isolating the 3rd scan in the analysis, indicating good reversibility. Moreover, the anodic and cathodic peak separation mean is about 80mV, which is equivalent to the established gold baseline. The cyclic voltammetry was mostly a symmetrical cycle, which suggests that the electrode was able to be used for the electrochemical test.

TABLE 4B. Two-Sample T-Test and Cl: current red and current ox (nickel) Two-sample T for current red vs current ox N Mean StDev SE Mean Current red 18 1.401 0.397 0.094 Current ox 18 1.461 0.370 0.087 Difference = µ (current red) - µ (current ox) Estimate for difference: -0.060 95% Cl for difference (-0.321, 0.200) T-Test of Difference = 0 vs ≠): T-Value = -0.47 P-Value= 0.640 DF=33

However, a second redox pair becomes evident after multiple scans when using the nickel electrode material. This could be indicative of the initial half-reaction on a forward scan of Ni ⇆ Ni²⁺ + 2e⁻, a potentially detrimental effect in biosensor applications.

Example 2

The performance of existing biosensors utilizing precious metals such as gold (Au) was again compared with the performance of the ruthenium-based alloys disclosed herein, using identical test method parameters as those set forth in Example 1 above.

In Example 2, gold (Au), palladium (Pd), ruthenium (Ru) and nickel (Ni) were initially compared against the Nernstian ideal of 59 mV, which is desirable and indicative of electron transfer kinetics that are beneficial for amperometric sensing applications. As shown in FIG. 14 , gold, palladium, ruthenium, and nickel all exhibit redox reactions in the system evaluated. Generally, peak potential separations were close to the Nernstian ideal of 59 mV. Ruthenium and ruthenium-based alloys show good properties in regards to the ferro/ferricyanide mediator system, with improved maneuverability and electrochemical stability in the potential range evaluated over nickel.

Binary ruthenium alloys evaluated for performance and cost are shown in Table 5 below:

TABLE 5. Binary ruthenium alloys average current response (units in µA, 1 mM K₄Fe(CN)₆ analyte) Alloy At% Ru Evaluated 50-60 60-70 70-80 80-90 90-100 Performance notes RuAl 0-40, 60-100 - 1.2073 1.3144 1.2427 Best results at ~ 65% Ru RuNi 60-100 - 1.2759 - 1.3252 High background current RuW 0-40, 60-100 - 1.1613 - 1.1928 1.3532 Stable, repeatable RuCr 0-40, 60-100 1.1387 - - 1.1481 1.208 Stable, repeatable RuRe 60-100 - 1.3635 1.0787 1.0686 Higher background current

Binary nickel alloys evaluated for performance and cost are shown in Table 6 below:

TABLE 6. Binary nickel alloys average current response (units in µA, 1 mM K₄Fe(CN)₆ analyte) Alloy At% Ni Evaluated 50-60 60-70 70-80 80-90 90-100 Performance notes NiAl 60-100 - 0 - 0.88 Elimination of response in lower Ni compositions NiCu 60-100 - - - - Noisy, variable NiTi 60-100 0 1.683 - 0.6596 0.7878 Comparable to NiCr at Ni>80% composition NiCr 60-100 - - - 0.764 Repeatable; appears quasi-reversible NiTa 60-100 - - - - Weak, variable response

By way of comparison, Table 7 below shows pure gold (Au) and palladium (Pd) metals evaluated for performance and cost:

TABLE 7. Pure metals average current response (units in µA, 1 mM K₄Fe(CN)₆ analyte) Metal 90-100 at% Performance Notes Au 1.122 Stable, low background, reversible Pd 0.7217 Strong initial response, degrades after multiple scans

The average responses for the most promising binary ruthenium and nickel alloys tested, along with gold and palladium for comparison, is shown in Table 8 below:

TABLE 8. Average Ipa, Epa, Ipc, and Epc responses for most promising compositions Test Composition Avg. Ipa (µA) Avg. Epa (mV) Avg. Ipc (uA) Avg. Epc (mV) Au - 1.1228 -55 -1.0599 -143 RuW 60/40 1.1733 -37 -1.242 -120 RuW 80/20 1.125 -43 -1.1 -133 RuCr 60/40 1.1387 -47 -1.0002 -153 RuCr 80/20 1.0792 -60 -0.9873 -157 NiCr 80/20 0.764 -27 -0.3566 -120 NiTi 60/40 0.1683 97 0 0 NiTi 80/20 0.5493 33 -0.273 -50 NiAl 60/40 0 0 0 0 NiAl 80/20 0.88 -90 0 -123 Pd - 0.7237 -13 -0.2777 -118

Ruthenium-based alloys RuW and RuCr have shown good response relative to gold with compositions of greater than 50 at% ruthenium, as presented above and as shown in FIG. 15 .

Nickel-based alloys NiCr, NiTi, and NiAl have shown good response relative to gold with compositions of 80 at% nickel or greater, as presented above and as shown in FIG. 15 .

Thus, in view of the aforementioned Examples 1 and 2 discussed above, screening results indicate that non-precious metal alloys, including nickel and/or ruthenium (note that though ruthenium may be classified as a precious metal, in this context it qualifies as a candidate based on cost), advantageously comparable responses and performance to expensive pure metals such as gold and palladium. In addition, these non-precious metal alloys may further be optimized for corrosion resistance through the addition of titanium (Ti), tantalum (Ta), and chromium (Cr). Moreover, these non-precious metal alloys may further be optimized for cost through the addition of nickel (Ni), titanium (Ti), molybdenum (Mo), aluminum (Al), tin (Sn), etc.

Particular alloys that have responses comparable to gold and exceeding palladium or NiCr performance include RuW (Ru > 50%) and RuCr (Ru > 50%). Particular alloys that have responses comparable to NiCr include NiTi (Ni > 80 at%), NiAl (Ni > 80 at%), or NiRu (Ni > 50at %).

In addition, ternary alloys have been tested with the same combination of the above elements (e.g., RuCrW and RuAlNi). These ternary alloys have advantages when designing for certain material properties (e.g., ductility, corrosion resistance, heat resistance).

Example 3

The performance of biosensors utilizing a metal layer stack as an electrode was explored. A three-electrode test cell was made consisting of the test material as a working electrode, Ag/AgCl reference, and Pt coil counter electrode. In parallel, a target condition of ~1.9 ohms/sq was fixed and deposition parameters were adjusted to meet the target. The electrodes were placed in a test fixture and connected to a BASi potentiostat and analyzer. The Cyclic Voltammetry (CV) test parameters are summarized in Table 9 below:

TABLE 9. Cyclic Voltammetry Test Condition Value KCl Concentration 0.1 M K₄Fe(CN)₆ Concentration 1 mM WE area 5.5 mm² Scan range -.250 mV to 600 mV Scan rate 50 mV Test Repeats 6

A RuW top layer was deposited on aluminum or silver base layers each of varying thickness. The resistance of each sample (run) was measured. The results are shown in Table 10 below.

TABLE 10. Run Al Thickness (nm) RuW thickness (nm) Resistance (ohms/sq) Al Strike 50 1.7 RuW Strike 30 53.8 1 30 10 3 2 30 30 2.8 3 50 10 1.7 4 50 30 1.6 Target 1.9 ohms 1.6 Target 1.9 ohms 1.8 Run Ag Thickness RuW thickness (nm) Resistance (ohms/sq) Ag Strike 50 0.84 RuW Strike 30 9.98 1 3 10 1.7 2 30 30 1.7 3 50 10 0.9 4 50 30 1 Target 1.9 ohms 10 1.8 Target 1.9 ohms 30 1.9

Thinner base metal layers (aluminum) with thicker RuW layers were explored along with other base layer materials (Cr, Mo, and W). The results are summarized in Tables 11-14 below.

A stacked material configuration should behave as parallel resistors, i.e. (1/Rt - 1/₁ +_1/R₂ ... 1/_(n)). Using resistance vs. thickness curves of Al and RuW, the parallel resistor model is confirmed experimentally below, and is an accurate predictor of sheet resistance in the range tested. A contour plot of resistance vs aluminum thickness and RuW thickness is shown in FIG. 16A for the Al resistor model and in FIG. 16B for the measured response. These results indicate that layer separation is significant, even with thin (100 angstrom) depositions.

TABLE 11. Aluminum (Al) Run Al thickness (nm) RuW thickness (nm) Resistance Parallel resistor model 1 10 0 8.99 2 10 50 6.73 6.73 3 20 50 3.80 3.8 4 10 75 5.91 5.87 5 20 75 3.52 3.52

TABLE 12. Chromium (Cr) Run Cr Thickness (nm) RuW Thickness (nm) Resistance Estimate Measured Resistance (ohms/sq) 1 50 0 11.7 2 30 50 11.55 11 3 50 50 8.30 7.84 4 30 75 9.31 9.01 5 50 75 7.08 6.84

TABLE 13. Molybdenum (Mo) Run Mo Thickness (nm) RuW Thickness (nm) Resistance Estimate 1 50 0 3.2 2 30 50 4.48 3 50 50 2.85 4 30 75 4.10 5 50 75 2.69

TABLE 14. Tungsten (W) Run W Thickness (nm) RuW Thickness (nm) Resistance Estimate 1 50 0 3.3 2 30 50 4.68 3 50 50 2.99 4 30 75 4.27 5 50 75 2.77

Example 4

Chromium and RuW samples were prepared as in Example 3. The electrochemical properties were then measured as done in Example 2. The results are shown below in Table 15. All conditions exhibited a redox response comparable to Au electrode performance. The Cr/RuW configuration should be feasible for a reduced overall material sheet resistance. FIG. 17 is a cyclic voltammetry graph showing Cr/RuW over a range of thicknesses (the same as in Table 15). Electrode performance in 1 mM ferro/ferricyanide solution is similar to Au.

TABLE 15. Sample Measured Cr thickness (nm) Measured RuW thickness (nm) Measured Resistance (ohms/sq) Ipc (avg.) Epc (avg.) Peak separation Cr 1 56 12.87 Oxidation only Cr 2 29.3 49.6 11 4.945 179.83 64.3 Cr 3 47 49.7 7.84 5.184 180.33 32.7 Cr 4 29.3 74.5 9.01 5.495 175.67 71.7 Cr 5 47.6 75.4 6.84 5.45 177 65.33 Au ref. n/a n/a n/a 4.869 166.67 95.17

Finally, FIG. 18 is a cyclic voltammetry graph showing electrodes with an electrode layer made of Au, RuW, and RuW/C. The addition of carbon increased the potential window of the electrode.

The electrodes of the present disclosure have been described as being useful in biosensor/electrode type applications. However, it should be understood they may be useful in any sensor article or device that, as a result of a chemical interaction or process between an analyte and the sensor, transforms chemical or biochemical information of a quantitative or qualitative type into an analytically useful signal. For example, it is contemplated that the multi-layer electrodes disclosed herein may be included in articles useful in automotive, indoor air quality (IAQ), food, agriculture, medical, water treatment, environmental, industrial safety, utilities (e.g., gas, electric), petrochemical, steel, military, and aerospace applications and markets.

It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims. 

1. A method for forming a sensor for measuring an analyte, comprising: forming a first electrode on a substrate, wherein forming the first electrode comprises: forming a first base layer on the substrate, the first base layer comprising a conductive metal, a conductive metal alloy, or carbon; and forming a first electrode layer on the first base layer, the first electrode layer comprising ruthenium metal, a ruthenium based metal alloy, nickel metal, or a nickel based metal alloy, wherein the first base layer comprises different elements than the first electrode layer; and forming a second base layer on the first electrode layer.
 2. The method of claim 1, wherein the first base layer is formed from a plurality of sublayers.
 3. The method of claim 1, wherein the first base layer comprises diamondlike carbon.
 4. The method of claim 1, further comprising: forming a second electrode on the substrate, wherein forming the second electrode comprises: forming a second multi-layer stack on the substrate; and forming a reagent layer on the second multi-layer stack, wherein the first base layer, the first electrode layer, and the second base layer together form a first multi-layer stack, and wherein the second multi-layer stack of the second electrode has a same structure as the first multi-layer stack of the first electrode.
 5. The method of claim 1, further comprising: forming reaction chamber in the substrate that is in contact with the first electrode.
 6. The method of claim 1, further comprising: patterning the first base layer, the first electrode layer, and the second base layer to form a first multi-layer stack and a second multi-layer stack, wherein the first multi-layer stack forms the first electrode; and forming a second electrode by forming a reagent layer on the second multi-layer stack.
 7. The method of claim 6, further comprising: connecting the first and second electrodes to a measuring device, wherein the measuring device is configured to apply a biasing potential signal through the first and second electrodes when a fluid comprising the analyte is exposed to the first and second electrodes.
 8. A method for forming a sensor for measuring an analyte, comprising: forming a first electrode on a substrate, wherein forming the first electrode comprises: forming a first electrode layer on a substrate; forming a first base layer on the first electrode layer, the first base layer comprising a conductive metal or a conductive metal alloy, or carbon; and forming a second electrode layer on the first base layer, the second electrode layer comprising ruthenium metal, a ruthenium based metal alloy, nickel metal, or a nickel based metal alloy.
 9. The method of claim 8, wherein the first base layer comprises different elements than the first electrode layer.
 10. The method of claim 8, wherein the first base layer is formed from a plurality of sublayers.
 11. The method of claim 8, further comprising: forming a second electrode on the substrate, wherein forming the second electrode comprises: forming a third electrode layer on the substrate; forming a second base layer on the third electrode layer; forming a fourth electrode layer on the second base layer; and forming a reagent layer on the fourth electrode layer such that the second electrode operates as a working electrode, and wherein the first electrode operates as a reference electrode.
 12. The method of claim 11, wherein the third electrode layer comprises the same material and structure as the first electrode layer, wherein the second base layer comprises the same material and structure as the first base layer, and wherein the fourth electrode layer comprises the same material and structure as the second electrode layer.
 13. The method of claim 12, further comprising: forming reaction chamber in the substrate that is in fluid contact with the working electrode.
 14. The method of claim 12, further comprising: connecting the working and reference electrodes to a measuring device, wherein the measuring device is configured to apply a biasing potential signal through the working and reference electrodes when a fluid comprising the analyte is exposed to the first and second electrodes.
 15. A method for measuring an analyte in a fluid sample comprising: providing a sensor configured to receive and measure a concentration of the analyte based on a reduction/oxidation reaction of the analyte at a working electrode of the sensor; applying the fluid sample to a reaction chamber of the sensor, the reaction chamber being fluidly coupled to the working electrode and a reference electrode; and receiving a signal output from the sensor indicating the concentration of the analyte, wherein the reference electrode is formed from a first multi-layer stack comprising a first base layer and a first electrode layer, wherein the first base layer comprises a conductive metal, a conductive metal alloy, or carbon, and wherein the working electrode comprises a reagent layer arranged on a second multi-layer stack, the second multi-layer stack having a same structure as the first multi-layer stack.
 16. The method of claim 15, wherein the first electrode layer is made from different materials than the first base layer.
 17. The method of claim 15, wherein the first base layer has a higher electrical conductivity than the first electrode layer.
 18. The method of claim 15, wherein the first and second multi-layer stacks are arranged on a same substrate.
 19. The method of claim 15, wherein the signal output from the sensor is based on a biasing potential signal applied to the working electrode and the reference electrode as the reduction/oxidation reaction occurs at the working electrode.
 20. The method of claim 15, wherein the first multi-layer stack and the second multi-layer stack are formed by: forming the first base layer and the first electrode layer on a substrate; and removing a portion of the first base layer and the first electrode layer from the substrate such that the first multi-layer stack and the second multi-layer stack are spaced apart from one another on the substrate and are formed from the same first base layer and the first electrode layer. 